Warning: Elementary level math involved.

Everytime you get dyno results people say "figure a xx percent loss from the crank to the wheels." I know this number differs from car to car, as well as dyno to dyno. My question is this:

If a baseline run in an S2000 gets 200 WHP for an engine rated at 240 at the crank, it means you're suffering about a 17% loss from the crank to the wheels.

If you mod the same car, and re-dyno it at 300 WHP (hypothetical situation), do you still assume a 17% loss? (360 at the crank).

The way I see it, we already know that it takes 40 HP to "drive" the car (i.e. parasitic losses, friction, driveline, clutch, etc. etc.) If none of that changes when you mod the car (assume no supercharger), shouldn't we just assume that it still takes only 40 HP (340 HP?) Why would you all of a sudden be using 60 HP to do a motion that we already know only took 40 HP?

TIA to anyone who can help shed some light on this subject

Mods: If this belongs in UTH, I apologize profusely, please move if nec.

 

I suppose a good way to look at this would be to understand that it takes more power to move an object more quickly.

If you engine is more powerful it will take more of that power to move the drivetrain components at the faster acceleration because of the higher HP engine.

Second, you lose more in friction losses the higher the torque on drivetrain parts. If you have 70 ft-lbs on a gear it will absorb a certain amount of power to turn the gear. When you put 150 ft-lbs on the gear it will absorb a larger amount of power to turn the same gear (heat loss and mechanical deformation).

Since power is how fast work is being done, a faster car (i.e. more horsepower) will absorb more power than the same car which takes more time to do the same work (acceleration).

 

It's all about the friction!!! Also the parasitic loss you are talking about is not a constant.

 

You'd have to look at how Honda originally arrived at their figure. Then, think about 17%...it sounds like a big percentage for drivetrain loss (but I could be wrong).

I'd think that the drivetrain loss would decrease(slowly) as the power increased; it would not be inversely proportional, there would be a curve to the line if it were graphed. Again, I could be way wrong, it's just what I suspect.

Barrett.

 

mr.B, could you care to elaborate a little more please? How does the friction increase with more power? Isn't the engine/trans/etc. doing the same thing from 0-9000 RPM regardless of how much power it's making?

I think AZS2000 was trying to explain this, but I've read his post 4 times and still not quite sure.

(I'm not being a smartass, I just really don't know.)

Ok, on the 5th reading, I'm realizing that yes, it would suck more power to accelerate a part (e.g. driveshaft) at a higher rate of speed with more power.

 

The faster you rub your hands togather the more friction that motion creates. I am just trying to come up with a good analogy

 

Maby that was a bad one

 

Ha ha... I was JUST thinking about that analogy, but I was using "rub your hand on a table."

The analogy doesn't work for me because a car with more HP will still run through the exact same engine speeds as a car with lower HP (0-9000 RPM), it will just be running through those RPM faster. The friction it will be building will be the same... it will just be building the friction faster. Although when thinking about it, I suppose it could take more HP for the engine to overcome the more rapid acceleration of the same friction. Oh wait... did I just answer my own question? Thanks for nothing guys

 

a dyno calculates hp by measuring the acceleration of two drums of known mass. to increase the hp you have to spin the drums faster and increase the acceleration. the faster you can spin the drums, the harder the engine is working and the more WORK the engine does and the greater your losses will be.

the analogy with the hands is a good one.... the faster you rub your hands, the more work you are doing and the greater the friction (heat). Like starting a fire with two sticks, just rubbing two sticks together slowly produces no heat.... but gradually increase the speed (acceleration) to a point and you can create smoke then fire.... that's a lot of friction (ie wasted energy going to heat)!!!!

Rule of thumb parasitic losses are anywhere from 15% - 25%.... 25% would be something like a camaro with an auto tranny....

 

I believe that slow and fast produce exactly the same friction and heat per cycles, if all other conditions are identical. BUT the rate of heat production (heat/unit time) is different because the time for, say 20 cycles (with the drivetrain or the hands) is substantially reduced when things happen faster. The rate of heat production is higher for fast, because the time units (denominator) are diminished, while the coefficient of friction and heat per cycle stay the same. For whatever the hell it's worth, that's what I think.